Recently, Metal-Oxide-Semiconductor (MOS) devices are widely used in digital circuits and analog circuits per their advantages on high switching speed, low power loss, and low cost. One of the most common MOS devices by far is the Metal-Oxide-Semiconductor Field Effect Transistor (MOSFET). When MOSFET is turned on, current can flow through the device, and when MOSFET is turned off, current can be blocked. A MOS unit comprising a plurality of MOSFETs can be employed for switching, regulating voltage, regulating current and so on.
MOSFET comprises three terminals, gate (G), source (S) and drain (D). MOSFET can be turned on or off via controlling gate voltage. Power loss is produced during MOSFET's ON period and OFF period.
Generally speaking, the power loss produced by MOSFET comprises three parts. The first part is driving loss which is generated by charging and discharging a gate capacitor. Driving loss is related with gate charge Qg, gate voltage VDD and switching frequency Fsw. The second part is switching loss which is related with input voltage Vin, turning-on time period Ton, turning-off time period Toff, load current IO and switching frequency Fsw. The third part is conduction loss which is caused by current flowing through MOSFET when the MOSFET is on. Conduction loss is related with on-resistance Rdson of MOSFET.
For apparatus comprising a MOS unit, switching loss and conduction loss are dominant in total power loss of the MOS unit when the apparatus operates under a heavy load. Conventional method to reduce switching loss and conduction loss is to decrease the on-resistance Rdson of the MOS unit, e.g., increasing parallel MOS devices to reduce the total conduction impedance of the MOS unit.
But gate capacitance increases when on-resistance Rdson decreases. As a result, driving loss increases. More specifically, when the number of parallel MOS devices increases, gate area and gate charge increases accordingly, and driving loss increases. However, driving loss is dominant in total power loss of the MOS unit when the apparatus works at light load and there is a conflict for decreasing power loss or increasing efficiency between light load and heavy load.
Some apparatuses, e.g., television, air-conditioning and so on, may maintain standby mode or low power mode within a long time period, and the internal MOS unit may work at light load within a long time period accordingly. As a result, a control method or apparatus for MOS unit is needed to achieve lower power loss at both light load and heavy load.